Cells receive and interpret signals from their extracellular environment through biochemical cascades known as signal transduction pathways. These pathways control many aspects of cell growth and differentiation. Mutations activating these processes are often found in human cancers. An important example of this is found in the Wnt signaling pathway. Wnts are secreted glycoproteins commonly used during invertebrate and vertebrate development that function through a highly conserved signaling apparatus. Recessive mutations in negative regulators of the pathway and gain-of-function mutations in positive effecters play a causal role in several cancers. Our goal is to identify new Wnt signaling components, by studying Wingless (Wg), a well characterized Drosophila Wnt. In one approach, a genetic screen was performed which identified a gene, gammy legs (gam), which blocks Wg signaling when over expressed. Removal of gam activity also leads to loss of Wg signaling. gam encodes a protein with a putative nuclear localization sequence and a PHD finger. The molecular mechanism of Gam action in Wg signaling will be persued using genetic epistasis in flies and fly cell culture, identification of possible binding partners for the Gam protein and a systematic structure/function mutational analysis. Two predicted human genes that have signficant sequence similarity to gam have also been identified in the database. These genes will be cloned and analyzed to determine if they play an important role in Wnt signaling in human cell lines. The second approach takes advantage of the recent finding that incubating Drosophila cells with dsRNA corresponding to a particular gene specify targets that gene's mRNA for degradation. This technology will be used to screen potential fly homologs of several factors recently implicated in vertebrate Wnt signaling through protein-protein interaction screens. If reduction of a gene's expression causes Wg signaling to be effected in cell culture, then fly mutants lacking these genes will be isolated and analyzed for Wg signaling defects. Obtaining genetic confirmation of the importance of these putative Wnt signaling components will provide the confidence to pursue their study in the context of cancer biology and as targets for drug discovery.